This invention is directed to zinc oxide (ZnO) films for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), and photodetectors. More particularly, this invention is directed to ZnO films containing a p-type dopant for use in LEDs, LDs, FETs, and photodetectors wherein both n-type and p-type materials are required, for use as a substrate material for lattice matching to other materials in such devices, and for use as a layer for attaching electrical leads.
For some time there has been interest in producing II-VI compound wide band gap semiconductors to produce green/blue LEDs, LDs and other electrical devices. Historically, attempts to produce these devices have centered around zinc selenide (ZnSe) or gallium nitride (GaN) based technologies. However, these approaches have not been entirely satisfactory due to the short lifetime of light emission that results from defects, and defect migration, in these devices.
Recently, because ZnO has a wide direct bandgap of 3.3 eV at room temperature and provides a strong emission source of ultraviolet light, ZnO thin films on suitable supporting substrates have been proposed as new materials for light emitting devices and laser diodes. Undoped, as well as doped ZnO films generally show n-type conduction. Impurities such as aluminum and gallium in ZnO films have been studied by Hiramatsu et al. who report activity as n-type donors (Transparent Conduction Zinc Oxide Thin Films Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci. Technol. A 16(2), March/April 1998). Although n-type ZnO films have been available for some time, the growth of p-type ZnO films necessary to build many electrical devices requiring p-n junctions has to date been much slower in developing.
Minegishi et al. (Growth of P-Type ZnO Films by Chemical Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recently reported on the growth of nitrogen doped ZnO films by chemical vapor deposition and on the p-type conduction of ZnO films at room temperature. Minegishi et al. disclose the growth of p-type ZnO films on a sapphire substrate by the simultaneous addition of NH.sub.3 in carrier hydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10 mol % was used, secondary ion mass spectrometry (SIMS) confirmed the incorporation of nitrogen into the ZnO film, although the nitrogen concentration was not precisely confirmed. Although the films prepared by Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear to incorporate a small amount of nitrogen into the ZnO film and convert the conduction to p-type, the resistivity of these films is too high for application in devices such as LEDs or LDs. Also, Minegishi et al. report that the carrier density for the holes is 1.5.times.10.sup.16 holes/cm.sup.3, which is considered to be too low for use in commercial light emitting devices or laser diodes.
Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producing thin films on substrates by doping IIB-VIA semiconductors with group VA free radicals for use in electromagnetic radiation transducers. Specifically, Park et al. describe ZnSe epitaxial thin films doped with nitrogen or oxygen wherein ZnSe thin layers are grown on a GaAs substrate by molecular beam epitaxy. The doping of nitrogen or oxygen is accomplished through the use of free radical source which is incorporated into the molecular beam epitaxy system. Using nitrogen as the p-type dopant, net acceptor densities up to 4.9.times.10.sup.17 acceptors/cm.sup.3 and resistivities less than 15 ohm-cm were measured in the ZeSe film. However, the net acceptor density is too low and the resistivity is too high for use in commercial devices such as LEDs, LDs, and FETs.
Although some progress has recently been made in the fabrication of p-type doped ZnO films which can be utilized in the formation of p-n junctions, a need still exists in the industry for ZnO films which contain higher net acceptor concentrations and possess lower resistivity values.